Post on 22-Jan-2023
International Journal of Recent Technology and Engineering (IJRTE)
ISSN: 2277-3878, Volume-8, Issue-1, May 2019
728
Published By:
Blue Eyes Intelligence Engineering
& Sciences Publication Retrieval Number: F2887037619/19©BEIESP
Abstract: The process of separation, extraction, and
purification in obtaining rare earth metals from the minerals is a
tedious work due to the similarity of the rare earth metals in
chemical and physical properties. Besides designing the rare earth
extraction sequence and removing the impurities, the selection
step for the type of extractant is also a crucial phase. There are
numerous possible combinations of extractant – metal in rare
earth separation, ranging from single extractant system to
multiple extractant systems. This paper aimed to guide and
suggest the best extractant, subject to the leaching condition and
the targeted end product. Each type of extractant is revised
separately in providing the mechanism and interaction of the rare
earth metals. This paper also evaluate the past and current trends
of these extractants besides identifying their strong point and the
drawbacks.
Index Terms: DEHPA, EHEHPA, extractant, rare earth,
solvent extraction
I. INTRODUCTION
From a mobile phone to the most advanced instrument in
medicine and defense, it would be impossible to uphold the
current modernize life without rare earth (RE) metals. With
the increasing demands from the industries, the extraction
and separation of RE become more and more important.
Despite the fact that they are not completely rare, United
States Department of Energy [1] and European Commission
[2] have labelled the supply of these REs as ‘critical’ for their
limited resource.Due to the uncertain market of RE, the
Europe researchers have been desperately introducing
various measures such as trade policies, industrial
adjustment, and budget allocations in the member states in
securing the RE supply [3].
The RE comprises 15 elements from the lanthanide series,
plus yttrium, and scandium. These REs can be categorized by
a variety of physical properties depending on the applications
of the RE as functional materials such as 1) the double-salt
solubility (light rare earth [LRE] and heavy rare earth
[HRE]); and 2) the extractability with acidic extractants
(LRE, medium rare earth [MRE], and HRE) (Table 1). For
the characterization of RE based on double salt solubility, RE
are converted to double sulfate salt precipitated,
(NaRE(SO4)2×H2O) and tested for their solubility in the
water. The LRE from this category showed a very low
Revised Manuscript Received on May 23, 2019
Nurul Ain Ismail, Earth Resources & Sustainability Center, Universiti
Malaysia Pahang, Pahang, Malaysia
Mohd Aizudin Abdul Aziz,, Faculty of Chemical and Natural Resources
Engineering, Universiti Malaysia Pahang, Universiti Malaysia Pahang,
Pahang, Malaysia
Mohd Yusri Mohd Yunus, Earth Resources & Sustainability Center,
Universiti Malaysia Pahang, Pahang, Malaysia.
Anwaruddin Hisyam, Faculty of Chemical and Natural Resources
Engineering, Universiti Malaysia Pahang, Universiti Malaysia Pahang,
Pahang, Malaysia
solubility in water. In the classification based on the
extractability with an acidic extractant, the lanthanide group,
from lanthanum to neodymium, is known as LRE. MRE
consists of samarium, europium, and gadolinium, while HRE
is a group from terbium to lutetium. Yttrium usually belongs
to the HRE group, whereas scandium does not belong to any
group. Scandium has significantly different chemical
properties compared to the rest of the RE, and it is rarely
found in the minerals. The latter category is more common to
be used in the literature.
Table 1: Classification of REMs
Double salt solubility
Light Rare Earth Lanthanum (La), cerium (Ce),
praseodymium (Pr), neodymium
(Nd), promethium (Pm), samarium
(Sm) and europium (Eu)
Heavy Rare Earth Gadolinium (Gd), terbium (Tb),
dysprosium (Dy), holmium (Ho),
erbium (Er), thulium (Tm),
ytterbium (Yb), lutetium (Lu) and
yttrium (Y)
Extractability with acidic extractant
Light Rare Earth La, Ce, Pr, and Nd
Medium Rare
Earth
Sm, Eu, and Gd
Heavy Rare Earth Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y
Considering their similarity in the chemical and physical
properties, RE usually exist in mineral form, which make the
separation process more challenging and expensive. The
separation of RE into individual RE is crucial before they
could be introduced into various products. Therefore, RE
metallurgy is considered critical, and it has consistently been
studied by researchers since the 1970s. Their aim is to
improve the purity of the end product, whilst reducing the
cost, time, and waste at a minimum level. RE metallurgy is
designated to the continuous line of unit operation, dealing
with mining, physical beneficiation, chemical treatment,
separation, and refining process (Fig. 1). Overall, the
combination of the processes able to transform the ore to a
combination or individual metal that is either a transition or
an end product converting them into metal, alloy, or another
compound. This paper focuses on exploring the key factor
contributing to the selection of the extractant in the separation
process, especially via solvent extraction.
Selection of Extractant in Rare Earth Solvent
Extraction System: A Review
Nurul Ain Ismail, Mohd Aizudin Abd Aziz, Mohd Yusri Mohd Yunus, Anwaruddin Hisyam
Selection of Extractant in Rare Earth Solvent Extraction System: A Review
729
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& Sciences Publication Retrieval Number: F2887037619/19©BEIESP
II. SEPARATION PROCESS IN RARE EARTH
METALLURGY
The central point of RE separation process is to convert the
RE concentrate to a RE mixture which is either the end or an
intermediate product for the following production of the
individual RE. Usually, the process requires one or more reagent to break down the ore and leach the RE into a
solution. Once all the metal is in the solution, the separation
process could be done either by solvent extraction (SX),
ion-exchange, fractional precipitation, fractional
crystallization or chemical precipitation. Type of minerals,
the concentration of the RE in the concentrate and targeted
end product are the main aspects need to be considered before choosing the separation process.
Mining Bench-Cut,
Open PitCrushing, Grinding
Chemical & Steam
ConditioningChemicals
Froth Floatation
Cleaning
(Thickening,
Filtering, Drying)
Carbonate
Minerals
Phosphate
Minerals
REE Chloride
Fusion Cake
Calcination
Oxide Minerals
Tailings
REO ~60%
High Temperature
Gases
Chlorination
Volatile Chloride
Oxide Minerals
Washing/
Dissolution/
Selective
Precepitation
Residue
Mixed REE Ionic
Solution
REE Metals
Mischmetals
High Temperature
Metallothermic
Reduction
Solvent Extraction
Hot Acid / Alkaline Leaching or Baking
REE Compounds
(Oxides, Chlorides, Fluorides, Carbonates)
Molten Salt
Electrolysis
Min
ing
Ph
ysic
al B
ene
ficia
tio
nC
he
mic
al T
rea
tme
nt
Se
pa
ration
Red
uction
/ R
efin
ing
/ M
eta
l M
akin
g
Hydrometallurgy
Pyro
meta
llurg
y
Fig. 1 Common rare earth metallurgy processing route
International Journal of Recent Technology and Engineering (IJRTE)
ISSN: 2277-3878, Volume-8, Issue-1, May 2019
730
Published By:
Blue Eyes Intelligence Engineering
& Sciences Publication Retrieval Number: F2887037619/19©BEIESP
SX is the most commonly used technique in RE separation,
especially in the industries due to its quick extraction process,
large separation capability, and high extraction capacity.
Apart from that, techniques such as fractional precipitation,
ion exchange, and fractional crystallization were difficult to
optimize due to the small difference in basicity [4]. Identical to ion exchange, SX incorporate the selective migration of a
metal ion within two solutions. In the liquid-liquid extraction
process, RE in the aqueous medium is attracted and captured
by the extractant (in the organic phase) due to the chelating
properties.
A. Fundamental of Solvent Extraction
Basic SX routine comprise extraction and stripping, and
the process can be expanded to scrubbing, washing, and
purification stages depending on the process requirement. An
aqueous phase usually consists of an acid load with rare earth
metals (REMa + REMb), which is mixed with an organic
phase or “extractant” in the extraction stage. The REM of
interest is denoted by REMa, whereas the others are denoted
by REMb. Due to the REs’ abilities to separate themselves
within aqueous and organic medium, some RE are carefully
moved into the organic phase. The extracted RE depends on
the number of H+, and the highest number separates the
lanthanide series further to the right part of the Periodic Table
[5]. The extraction step is repeated over and over to enhance
the separation of the REMa towards REMb. At the terminal
stage, the remaining REMb in the aqueous solutionis
channeled to raffinate and the extracted RE, that are mostly
REMa and some REMb, are later sent to the scrubbing
process. The scrubbing process is performed after the
extraction process and carried out by adding water or diluted
acid/base solution into the pregnant organic phase in
removing any impurities or unwanted metals such as REMb.
This process could improve the purity of the desired metals.
The organic phase contains only REMa that are stripped by
diluted acid or base, and then is recycled back to the
extraction stage. The pregnant stripping solution leaves a
stripping point with a high concentration of REMa and it is
recovered as REMa oxide after a series of downstream
processes. Fig. 2 shows the common process flow in a RE
extraction circuit.
REMa + REMa + REMa REMb + REMb
REMb + REMb + REMb + REMb + REMb + REMb
REMa + REMa + REMa
REMb + REMb
REMa + REMa + REMa
STRIPPING PROCESS
EXTRACTION
PROCESSSCRUBBING
PROCESSAqueous feed loaded with
REMa and REMb
Raffinate containing REMb
Scrubbing feed Scrubbing raffinate containing REMb
Stripping feed Pregnant stripping solution with REMa
Pregnant organic solution with REMa (major) and REMb (minor)
Pregnant organic
Rec
ycle
d organic solution so
lutio
n w
ith R
EMa
Fig. 2Basic process flow in solvent extraction containing extraction, scrubbing and stripping process
For a complete cycle of rare earth solvent extraction
(RE-SX), the process usually consists at least two out of three
types of SX separation as described below:
1) Pre-concentration SX: This process is vital for feed with a
low concentration of RE before separating the RE into RE
mixture or individual metals. Usually, an extractant with high
extraction capabilities is used [6]. 2) Removal SX: For feed with a mixture of other elements
or contaminants. This type of SX is commonly used as the
first SX stage for extraction of RE from ores. In this case, a
specific extractant need to be chosen which the distribution
coefficient for the RE is far from the other elements or
contaminants.
3) Individual SX: This process is applied to the high
concentration of REM solution to separate them into
individual metals.
Example on the type of SX system and the relationship
with the selection choice of extractant will be discussed in
Subtopic IV.
B. Distribution Coefficient and Separation Factor in SX
SX was first observed by Hopkins and Quill in their investigation for the solubility of various RE nitrates and
chlorides in different types of alcohol [7]. They succeeded in
separating neodymium and praseodymium nitrate with the
composition of 83:16 into 94:6 v/v after two stages of
extraction. Eight years later in 1941, Appleton and Selwood
introduced the concept of β in RE separation [8]. They
conducted a simple liquid-liquid extraction and found that the
βfor the separation of La and neodymium thiocyanate is 1.06.
They recommended that a continuous counter-current
extraction might help in improving the separation process,
compared to the traditional method that is fractional
crystallization.
Selection of Extractant in Rare Earth Solvent Extraction System: A Review
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Better separation of 2.14 for three extraction stages was
then achieved in separating lanthanum and neodymium
nitrate in n-hexyl alcohol. In 1949, Warf proved that solvent
extraction method which incorporates extraction and
stripping stages could be applied in RE separation. Over
99.5% of Ce(IV) was extracted from Ce(III) with tributyl phosphate (TBP) in the presence of nitric acid, which was
then stripped from its organic solution by the reduction to
Ce(III) using hydrogen peroxide [9].
During extraction process, when the REMs distribution
between the organic and aqueous solution achieved an
equilibrium state, the distribution ratio (D) of the REMs is
expressed as the following:
Eq. 1
[REMa(org)] and [REMa(aq)] are the concentration of targeted REM or REMa (Fig. 2) in the organic and aqueous
phase, respectively. Sometimes, D is also denoted as
distribution coefficient factor or distribution factor. The main
point of extraction is to shift the targeted REM from the
aqueous to the organic solution. The system that provides a
higher value of D for REM is the most preferred choice for
the process. If an extraction system comprises of various
REM species, D is considered as the total concentration of
REM in organic that is divided by the total concentration of
REM in the aqueous as the following:
Eq. 2
Another factor that has to be considered is the separation factor or β, which is the D of two REMs; REMa and REMb
under the same system condition:
Eq. 3
The higher βindicates that the separation between REMa
and REMb is easier. If the βis equal or close to one, the
separation between those two metals is difficult. Even if it is
possible, the system requires a large number of extraction
stages to separate the metals completely.
Currently, there is no universal rule to evaluate or to rank
the efficiency of an extraction system as it is a complex task.
The efficiency does not only depend on the D or β, but it also
comprises of other aspects, including physicochemical
property, kinetics rate, and interfacial phenomenon. Besides
that, it could become more perplex in industrial practice.
Provisionally, it is sensible to evaluate the efficiency of one
extractant by the value of D and β.
III. EXTRACTANTS IN THE RARE EARTH SOLVENT
EXTRACTION SYSTEM
In the SX system, there are two inlet stream: organic feed
and aqueous feed. An acid solution is commonly used as
aqueous feed, and an aqueous feed is a mixture of organic
solvent, extractant, and sometimes added with a modifier.
The extractant is in charge in accumulating the desired REM
into the organic phase for the the REM separation process.
Most of the extractants are viscous in nature; thus, a diluent is
required to dissolve the extractant as well as ensuring good
contact between the extractant and aqueous phase. The
various types of diluent include kerosene, n-hexane, benzene,
dichloromethane, and chloroform. Different diluent gives a
different value of equilibrium constant depending on the
extraction mechanism. Among the diluents, kerosene gives
the largest number of the equilibrium constant, which makes it was a preferable diluent in RE extraction [10]. Sometimes,
a modifier such as isopropanol [11], [12] and tributyl
phosphate (TBP) [13] have to be included to increase the
hydrodynamics of the system and to lower the possibility of
the third phase formation.
To achieve a good separation, process of synthesizing or
selecting the extractant must follow several criteria. Most
importantly, the extractant must possess at least one
functional group and a relatively long hydrocarbon chain or
benzene/substituted ring element. The functional group such
as P, N, O, or S acts as a metal complex with REM, and the
carbon chain is used to intensify extractant solubility in the solvent of choice. On top of that, a good extractant must have
positive selectivity towards the desired REM, excellent
chemical stability, low density, and viscosity, as well as large
surface tension. Some of these properties are crucial to
prevent emulsion for the extraction and stripping process
[10].
In the leaching stages (refer Fig. 1), dissolution of minerals
into an aqueous solution usually occur in a very high
concentration of acid. Wilson and his group explained in
detailed the character of the anion that is formed in the
aqueous which finally gives a significant hint on choosing the suitable extractant for the SX system [14]. If hydrochloric
acid (HCl) is used in the leaching stage, neutrally charged
extractant such as TBP and tri-n-octylphosphine oxide
(TOPO) are preferred. Chloride anion from the HCl is an
excellent inner-sphere ligand, and it could easily solvate the
metal salt and form an extractable, neutral-charged
complexes. However, the anion is more likely to exist as
chloridometalates at a relatively high concentration of
chloride. The most appropriate extractant to be used at a high
concentration of chloride is cationic extractant as the metal
salt can be extracted by the formation of outer-sphere
assemblies or ion pairs. On the occasion that sulphuric acid (H2SO4) is used, anionic extractant is the best candidate.
Compared to chloride, sulfate ion has a weak inner sphere
ligand, and an anion is required to achieve a neutrally charged
complex. Fig. 3 shows the simplified diagram of the selection
of extractant based on criteria of acid in leaching system.
For the past 50 years, researchers have discovered the
various type of extractants. However, only a small number of
extractants are used in industrial practice. The extractant
must act as a good separation agent between RE metals with a
low operational cost to satisfy the industrial expectation.
Di-2-ethyl-hexyl-phosphoric acid (DEHPA), 2-ethyl-hexyl-2-ethyl-hexyl-phosphonic acid (EHEHPA),
versatic acid, versatic 10, and Aliquat 336 are selected and
considered as practical for industrial application (Table 2)
[4], [15]. However, some industries formulated their
extractant and registered with a trade name, which makes it
difficult to determine the active component. This study will
discuss on these extractants regarding their chemistry
properties, best metal-extractant combination and also their
potential future development.
International Journal of Recent Technology and Engineering (IJRTE)
ISSN: 2277-3878, Volume-8, Issue-1, May 2019
732
Published By:
Blue Eyes Intelligence Engineering
& Sciences Publication Retrieval Number: F2887037619/19©BEIESP
For the past 50 years, researchers have discovered the
various type of extractants. However, only a small number of
extractants are used in industrial practice. The extractant
must act as a good separation agent between RE metals with a
low operational cost to satisfy the industrial expectation.
Di-2-ethyl-hexyl-phosphoric acid (DEHPA), 2-ethyl-hexyl-2-ethyl-hexyl-phosphonic acid (EHEHPA),
versatic acid, versatic 10, and Aliquat 336 are selected and
considered as practical for industrial application (Table 2)
[4], [15]. However, some industries formulated their
extractant and registered with a trade name, which makes it
difficult to determine the active component. This study will
discuss on these extractants regarding their chemistry
properties, best metal-extractant combination and also their
potential future development.
A. Single Extractant System
1) Organophosphorus acid
The most commonly used RE extractants from the
organophosphorus group are 2-ethylhexyl phosphoric acid
mono 2-ethylhexyl ester or also known as EHEHPA / P507
and di-(2-ethylhexyl) phosphoric acid or also known as
DEHPA / P204. Both extractants have colorless or yellowish
transparent liquid with non-polar properties. Peppard et al.
(1957) studied the properties and the application of DEHPA
as an extractant in separating REMs. DEHPA was accepted as a feasible extractant with a high number of D, besides
having the ability to lower the overall quantity of extraction
stages needed in the RE separation. They also discovered the
interaction of DEHPA, which majorly in dimer form towards
RE3+ ions as shown in Eq. 4, indicating that pH value has a
significant influence on the reaction [16]– [18]. Varying the
pH of the aqueous solution could affect the amount of REM
in the extraction, scrubbing, and stripping phase.
Eq. 4
(HL)2 refers to the dimeric species. (aq) is the aqueous
phase, whereas (org) is the organic phases.
DEHPA and EHEHPA are categorized as cation-exchange
extractants, which are similar to most of other
organophosphorus acid. The metal replaces the hydrogen ion
in the extractant and produces a soluble organic complex that
has a neutral charge. In excess, the combination of RE and
extractant complexes produce an 8-membered
pseudo-chelate (Fig. 4) by holding one of the hydrogens in
the dimer and releasing the other while replacing it with a
metal cation [14]. Sato compared both DEHPA and EHEHPA in various
condition. Based on the behavior of the REMs in HCl, the
study concluded that REs follow the cation exchange
mechanism in low acidity; and solvating mechanism in high
acidity [19]. Both extractants displayed higher extraction
efficiency for the metal with a larger atomic number in the
lanthanide series;
La<Ce<Pr<Nd<Sm<Eu<Gd<Tb<Dy(~Y)<Ho<Er<Tm<Yb<
Lu. Table 3 and Table 4 show the β for these two extractants.
The efficiency of the extraction process is parallel with the
atomic number of RE for any given organophosphorus acid.
The possible explanation for this is the attraction effect of the extractant anion, and the increase of the electrostatic force
causes the size of the cation to become smaller [4].
Fig. 3 Determination of extractant based on acid used in the
leaching stage
Sato in his paper stated that DEHPA demonstrates a good
separation ability for LRE compared to HRE due to the formation of complexes through the interaction between
DEHPA towards HRE, especially for Tm, Yb, and Lu [19].
Based on the current relative accumulation of β for each
REM, the Table 3 and Table 4 show an interesting pattern.
Eu> Gd>Sm and Tb>Gd>Eu are the three most difficult
elements to be separated in a mixture of REs for DEHPA and
EHEHPA system, respectively. The central part of the
lanthanides series has the highest difficulty for the separation
process, and the difficulty level lessens towards the end. This
might be the reason that samarium and europium could be
easily separated only after they are reduced into a divalent
state [4].
Fig. 5Formation of 8-membered pseudo-chelate by involving
phosphinic acids [14]
There are others few papers that evaluate the efficiency of
DEHPA and EHEHPA as an extractant in the same
environment [21]–[25]. Radhika et al. studied the separation
possibilities of LRE; La, Ce, Pr, and Nd and HRE; Tb, Dy,
Ho, Er, Yb, and Lu towards the three types of extractant,
which are DEHPA, EHEHPA, and Cyanex 272. They found
that the extraction efficiency is dependent on the pKa for each
extractant. EHEHPA> DEHPA> Cyanex 272 in H3PO4
medium [21]. The similar sequence of extraction is also
obtained by Soo Kim et al. through the use of HCl medium; although in some condition, the β are higher in DEHPA
compared to EHEHPA [23]. Contrary for La(III) in nitrate
medium, DEHPA is proven to have higher extraction
efficiency than EHEHPA
under all experimental
condition [22]. In another
Acid use in the leaching
stage
Hydrochloric acid, HCl
High concentration
of HCl
Best extractant: Cationic
extractant such as
Aliquat 336
Low concentration
of HCl
Best extractant: Neutrally charged
extractant such as TBP and
TOPO
Sulfuric acid, H2SO4
Best extractant: Anionic
extractant such as
DEHPA and EHEHPA
Selection of Extractant in Rare Earth Solvent Extraction System: A Review
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study, almost all REs in the study, which are Sm, Gd, Dy, and
Y, were extracted from the mixture by using 1 mol/L
DEHPA, while 0.1 mol/L EHEHPA shows the capability of
separating Dy-Y and Sm-Gd [24]. The 100% extraction
indicates that there is no occurrence of separation for
DEHPA. For the separation of HRE, Abreu and Morais
recommended the application of EHEHPA, rather than
DEHPA. Throughout the study, EHEHPA excelled in all
variables for both separation and stripping. EHEHPA showed
lesser interaction with REs, which makes the stripping
process easier [26]. The increase of the β in EHEHPA causes
the number of extraction stages to become lower. Contrarily,
based on the reduction ionic radius and increment of
coordination equilibrium constant, Kex across the lanthanide
group, Chen et al. summarized that the back extraction or stripping process was theoretically to be intricate due to
lanthanide contraction [27].
Table 2: Structures and pka of the Common Industrial Extractant
Commercial Name Full Name Chemical Structure pKa
Organophosphorus Extractants
P204
TOPS 99
Di-(2-ethylhexyl) phosphoric
acid (DEHPA) CH
H2C
C2H5
C4H9 O
P
O
OHOH2C
HCC4H9
C2H5
3.24
P507
PC 88A
Ionquest 101
2-Ethylhexyl phosphoric acid mono 2-ethylhexyl ester
(EHEHPA) P
O
OH
O
CH2
H2CC
H
C2H5
C4H9
HC
C2H5
C4H9
4.51
Carboxylic Acid Extractants
Versatic 10 Neodecanoic acid
COOH
7.33
Versatic acid Naphthenic acid R1
R2
R3
R4
(CH2)n COOH
6.91
Amine Extractant
Aliquat 336 Methyltrioctylamine chloride
N+
(CH2)7
(H2C)7 (CH)7
CH3
CH3
H3C CH3
Cl-
In the extraction process, lower D and extraction
percentage commonly due to the increase of acidity in the
aqueous phase. The high acidity comes from the additional
H+ that was produced from the cation-exchanging reaction.
Researchers have found a solution by replacing the
exchangeable H+ of the extractant with sodium ion (Na+) or
ammonium (NH4) through the saponification process. The
most commonly used saponification agents are ammonium
hydroxide (NH4OH) and ammonium bicarbonate
(NH4HCO3). However, there are several drawbacks in
saponification as the saponification agents are strong alkalis
which could produce a significant amount of heat that can
damage the equipment as well as affecting the overall
extraction process. Saponification could also produce a large
volume of wastewater containing ammonium.
International Journal of Recent Technology and Engineering (IJRTE)
ISSN: 2277-3878, Volume-8, Issue-1, May 2019
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& Sciences Publication Retrieval Number: F2887037619/19©BEIESP
Several researchers have studied the positive effects of
saponification in elevating the D [12], [28], [29]. The acidic
and saponified EHEHPA were used for the extraction of Nd
from chloride medium; as a result, Lee and his group found
that the saponified extractant could increase the D by 40%
[28]. For industrial level, Thakur and his co-workers had developed a mathematical model for the production of
neodymium oxide, Nd2O3 from the experimental data of 20%
saponified EHEHPA, and the model had succeeded in
obtaining over 5 kg of Nd2O3 (97%) with almost 85%
recovery [12]. Besides that, an experimental quantitative
co-extraction of Pr and Nd was done, and more than 99% of
Nd purity was achieved by using 50% saponified 0.15M
EHEHPA in the four stage of counter-counter extraction
setup [30]. Currently, much effort has been dedicated to the
search for an alternative to the saponification process. One of
the solutions is to introduce complexing agent into the
system, including ethylenediaminetetraacetic acid (EDTA) [31], pentetic acid (DTPA) [32], 2-ethyl-2-hydroxy butyric
acid [33], 8-hydroxyquinoline [34], lactic acid [35]–[38], and
citric acid [35], [36], [38]. A group of researchers explored a
system with the mixture of EHEHPA and
8-hydroxyquinoline in chloride medium to extract Pr(III),
Nd(III) and La(III). They found that at pH 3.6, the largest
synergistic enhancement factor occurred for Pr, Nd, and La(III) are 5.47, 3.37 and 2.86, respectively. These
complexing agents were unable to work alone; however, the
system could be improved and fit for industrial application as
the mixture could reduce the acidity in the stripping process
with the aid of the extractant [34]. The extraction of Ce(III)
and Pr(III) in the chloride medium with the addition of lactic
and citric acid was also performed and both metals showed
decent outcome for both D and β [35]. Yin et al. described the
effect of the complexing agent by showing that the system
with DEHPA and lactic acid in chloride medium was able to
double the extraction capacity up to 27.87 g/L, rather than
13.13 g/L in DEHPA-HCl system [37].
Table 3: Separation factorα of Ln(III)-HCl-HDEHP in RE extraction
Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ref.
La 2.14 2.28 2.43 11.8 26.3 44.6 71.1 101 125 212 319 414 425 [19]
Ce 1.07 1.14 5.2 12.3 20.9 33.3 47.2 58.4 99.1 149 193 199 [19]
Pr 1.06 5.16 11.5 19.5 31.1 41.1 54.7 92.7 139 181 186 [19]
Nd 4.86 10.8 18.3 29.2 41.5 51.3 87.1 131 170 175 [19]
Sm 2.23 3.75 6.02 8.55 10.6 17.9 27.0 35.1 36.0 [19]
Eu 1.69 2.70 3.83 4.74 8.04 12.0 15.7 16.2 [19]
Gd 1.60 2.26 2.80 4.75 7.15 9.30 9.55 [19]
Tb 1.42 1.76 2.98 4.48 5.83 5.90 [19]
Dy 1.24 2.10 3.16 4.11 4.22 [19]
Ho 1.70 2.55 3.31 3.41 [19]
Er 1.50 1.90 2.01 [19]
Tm 1.30 1.34 [20]
Yb 1.03 [19]
Table 4: Separation factora of Ln(III)-HCl-EHEHPA in RE extraction [19]
Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La 1.30 1.42 1.67 3.33 6.52 9.52 22.5 36.4 93.9 117 156 175 199
Ce 1.09 1.28 2.57 5.02 7.36 17.3 18.0 72.3 90.5 120 135 152
Pr 1.17 2.35 4.59 6.72 15.8 64.2 66.0 82.7 110 123 140
Nd 2.00 3.94 5.74 13.5 21.8 56.3 70.5 93.7 105 119
Sm 1.96 2.87 6.74 10.9 28.2 35.3 46.8 52.6 59.5
Eu 1.46 3.45 6.39 14.4 18.0 24.0 26.9 30.4
Gd 2.35 3.81 9.82 12.3 16.3 18.3 20.7
Tb 1.62 4.18 5.23 6.95 7.81 8.83
Dy 2.58 3.23 4.29 4.82 5.45
Ho 1.25 1.66 1.87 2.11
Er 1.33 1.49 1.69
Tm 1.12 1.26
Yb 1.13
Since its discovery, DEHPA has become the most studied
extractant, and the research is still ongoing. DEHPA has
dominated the extraction process for the industry in China,
and it was able to oust the prior extractant, TBP. For the current trend, the majority of the REs producers, especially in
China, have switched to EHEHPA due to its replaceable role
in the separation of the individual RE [27] as well as its
ability to strip RE in relatively lower acidity compared to
DEHPA [39]. DEHPA requires higher acidity level in the
stripping process which could affect the operational cost
significantly as 10 kg of 33% HCl and 2 – 3 kg sodium
hydroxide (NaOH) per kg are needed for the separation of
every kilogram of RE oxide. Table 5 shows the pros and cons
summary of these common extractants. In a single extractant system, it can be concluded that DEHPA is used to separate
LRE and MRE, while EHEHPA is used to separate individual
RE and HRE.
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However, the selection of the extractant is a complex
process as there is the need to evaluate the extractant’s effect
on starting materials, composition, and target product.
Table 5: Summary of common extractants
Advantages Disadvantages
DE
HP
A
Poor solubility in
aqueous solution
Relatively high
separation factor
compared to
EHEHPA
Better separation
in LRE and MRE
as compared to
EHEHPA
Strong bonding
capacity for HRE,
which resulted in the
difficulty to strip
HRE from loaded
organic
Relatively higher
acid and base
consumption for stripping compared to
EHEHPA
Large amount of
flammable diluent is
usually used
Requires
saponification
process
EH
EH
PA
Low consumption of
chemical reagent
and waste
discharge
Better separation
of HRE
Ability to
separate
individual RE
High acidity is
needed in stripping
some of the HRE
Large amount of
flammable diluent is
usually used
Requires
saponification
process
HA
Low cost
Easy preparation
Able to extract
high purity of
yttrium
Great solubility in
aqueous solution resulting in extractant
loss
Y extraction process
is done under high pH
value, which leads to
emulsification
Composition of HA
could be easily
changed during the
extraction process
Ali
qu
at
336
Efficient for LRE extraction
The extraction can
only be carried out in
high concentration of
nitrate system
2) Carboxylic acid
The early phase of SX development uses carboxylic acids
due to their attractive price, availability, and ease of use for
the separation of RE. Various type of carboxylic acid
extractants are currently available in the market. However, naphthenic acid (HA) and neodecenic acid (Versatic 10)
acids are the leading extractants in RE separation, especially
for yttrium. Preston conducted an extensive study on the
extraction of more than 30 metal cations, including RE by
using HA, Versatic 10, 2-bromodecanoic, and 3,
5-diisopropylsalicylic acids. The REMs of choice were from
LRE (La3+, Ce3+, and Nd3+), MRE (Gd3+), and HRE (Ho3+,
Yb3+) group [40]. These REs could be easily transformed into
complexes, which are predominantly ionic in nature. HA and
Versatic 10 able to extract REs in higher pH (~pH 4 -5)
compared to the other two acids. The position of yttrium is also varied depending on the type of extractant in use. For
Versatic 10 in xylene medium, the order is La< Ce< Nd<
Gd< Y< Ho< Yb, whereas HA has the order of La< Ce< Y<
Nd< Gd ~ Ho ~ Yb [40].
Preez and Preston continued the study for La, Gd, and Lu
in nitrate, and some cases in sulfate system. They reported
that the extraction of REs in carboxylic acid is depended on
their atomic number [41]. This statement indicates that there
is an explicit relationship in the steric bulk of the carboxylic
acid molecule that is represented by the steric parameter (Es)
of the substituent alkyl group. In sterically hindered
carboxylic acid like Versatic 10, the value of pH0.5 has resulted in the reduction from La to Lu and caused Y to
behave like MRE. For straight-chain and less sterically
hindered carboxylic acids, the value of pH0.5 is decreased
from La until MRE, and it gradually rises at Lu, which caused
Y to have similar behavior as LRE. Eq. 5, Eq. 6 and Eq. 7
show the equation for RE extraction in highly steric
hinderence complex and low steric hinderence for LRE and
HRE. Apart from Y, carboxylic acid extractants have poor
selectivity towards other RE due to their large complex
species that can coexist in the immiscible water phase. The
carboxylic acid extractant is quite stable in a hydrocarbon solvent; however, in low polarity solvent, it has the tendency
to associate and produce more complex speciation as shown
in Fig. 6 [14].
Eq. 5
Eq. 6
Eq. 7
Fig. 3Formation of neutral complexes during the extraction
of M2+ cations by hydrophobic carboxylic acids (Y = H2O or
RCO2H) [14]
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A study by Singh et al. discussed on the location of Y,
specifically on the effect of the structural changes in
carboxylic acid extractants [42], and the findings are similar
to the observation by Preez and Preston in 1992 [41]. Both
papers agreed on the usage of Es for quantitative means in
representing steric hinderance of carboxylic extractants. If the value of Es<1 is achieved, Y is inclined to behave more
like LRE group, and it will be extracted along with the group.
On the other hand, if Es>2, Y is positioned with MRE. These
verdicts, however, with the assumption that RE in mixture
behaves as the same as the individual.
Carboxylic acid extractant serves a practical use for RE
processing as it is relatively cheap compared to the other
common extractants besides being readily available. Besides
that, carboxylic acid extractant has been used in a
commercial process for various purposes, for instance,
Thorium Ltd. In England and Molycorp processing plant in
Colorado, they have been using HA to purify yttrium. In Thorium Ltd processing plant, a high purity of yttrium oxide
(Y2O3), the percentage of 99.999% was achieved from the
47% Y2O3 concentrate [4].
3) Amine
Amine extractant consist of primary up to quaternary
ammonium is very efficient in the separation of radioactive
elements, non-ferrous metals, and REs [43]. For REs, amine extractant works well with LRE. When amine extractant is
used with aqueous soluble amino carboxylic acid, HRE will
be extracted the most. Therefore, this type of extractant has
gained the attention of researchers. Coleman reported on the
effect of solution acidity on molecular species in organic and
aqueous phases for the separation and recovery of REM in
the primary amine system. The suggested route for separation
by amine extractant was either anion exchange mechanism or
adduct formation as shown in Eq. 8 and Eq. 9, respectively
[44].
Eq. 8
Eq. 9
R is an alkyl group and X is an inorganic anion.
As the potential of primary and tertiary amine in RE
separation was revealed by the papers published by Bauer
[45], [46], and in 1971 Baeur and Lindstrom [47] extend the
work to the Aliquat 336 in the presence of chelating agents
such as DTPA, HEDTA and
1,2-diaminocyclohexanetetraacetic acid (DCTA). Aliquat
336 has the highest βwith Socal 355L compared to other common water immiscible diluents such as toluene, xylene,
and ethyl ether. It was verified that Aliquat 336 in nitrate
system is a good extractant in the extraction of La-Pr, Pr-Nd,
and Nd-Sm with the βof 93, 5.4, and 11.6, respectively. These
values were higher in comparison to EHEHPA and DEHPA
in chloride system [19]. The limitation of Aliquat 336 system
is the dependency of the extraction efficiency on the amount
of inorganic nitrate in the solution. The formation mechanism
of the extractable complexes could only occur if the number
of nitrate ions in the solution is high. Cerna et al. in their
paper mentioned that the high concentration of nitrate ion of
4 to 8M could reduce theβ of the lanthanide series with
increasing atomic number [48]. The high concentration of
nitrate is necessary in the D of the system as it can provide the
ease of penetration into the anion exchanger due to the decrease of net charge. Apart from that, ion dehydration from
the partial stripping of the hydration sheath in the RE ions
could also contribute to the rising number of extractable
complexes. The hydration is higher in LRE, and it can be
steadily reduced along the lanthanide series [49].
The application of these extractants is not limited to
mineral hydrometallurgy. The same method could also
extract and separate RE from electronic waste, in particular
for the extraction of Pr and Nd from the permanent magnet.
Preston stated that the separation of Nd from magnet could be
achieved by using counter current solvent extraction process
with Aliquat 336 extractant in the nitrate medium [50]. Besides Aliquat 336, multiple studies have proven the
capabilities of other extractants such as Cyanex 272,
DEHPA, EHEHPA, Cyanex 921, Cyanex 301, Cyanex
302, and Alamine 336 in extracting and separating Nd and
Pr from pregnant leaching solution and synthetic solutions
[28], [51]–[55]. Among these extractants, DEHPA and
EHEHPA display better extraction efficiency due to their
high selectivity when extracting REs (La, Ce, Nd, Pr, Y) from
aqueous solutions.
B. Binary Extractant System
Currently, researchers are exploring new extractant,
extraction system, or equipment in improving the separation
process. Since the last decade, various extractant mixtures
were introduced in the search of better extractant, including a
combination of alkaline–acidic extractants [56]–[58],
acidic–acidic extractants [39], [59], [60], and neutral
organophosphorus-acidic extractants [61]. These binary
extractant systems could improve the extraction capability
and selectivity as well as enhance the solubility of the metal-extractant complexes in the organic solution, avoid
probability of emulsification and formation of the third
phases [61].
In the acidic-acidic extractants system of RE separation,
the synergistic interaction of EHEHPA with DEHPA are
among the most discussed extractants [39], [62]–[68]. This
system was introduced to overcome the limitations of both
DEHPA and EHEHPA that are shown in Table 5 through the
combination of extractant system which has low acidity in the
stripping process and high extraction efficiency for the
extraction of RE. This system demonstrates a very promising extractant as it has better βcompared to their individual
element (Table 6), besides exhibiting the highest
extractability among other mixture of the system;
EHEHPA/DEHPA> EHEHPA/Cyanex 301>
EHEHPA/HHEOIPP> EHEHPA/Cyanex 302 >
EHEHPA/Cyanex 272 [62]. Although there were studies
done in proving the synergistic effect of this system [39],
[63], [64], [69], it is still nascent. In a study that investigated
the extraction of Y, Nd, and Dy, Mohammadi and his
co-workers found that pure extractants have better β, except
for the extraction of Y from Dy [39]. Similarly, Zhang et al.
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found that the maximum βfor Ce/La in the binary system
without the addition of complexing agent is 3.35, which is
lower than those in the EHEHPA with the value of 5.84 [70]
and DEHPA with the values of (4.09, 3.92) [71].
Another important extractant that is commonly used in RE
separation is bis-2,4,4-trimethylpentyl phosphinic acid or also known as Cyanex 272 that was developed by Cyetch
Industries Inc. Cyanex 272 has higher pKa than DEHPA and
EHEHPA with the values of 6.37, and it has larger steric
hinderance because of the existence of methyl groups [72]. In
the previous study, Cyanex 272 showed wonderful β and
selectivity between adjacent RE and low extraction acidity,
which makes the stripping process easier. However, Cyanex
272 is costly [73], and it could easily form an emulsion in a
certain condition [73] as well as unable to separate few RE
couples cogently [72]. The binary system of Cyanex 272 and
EHEHPA was investigated by several researchers [15], [66],
[74]. Xiong and his collegues studied the extraction of Yb towards this mixture and verified that the occurrence of
synergism even for the HRE pairs [74]. However, the data for
HRE were insufficient as the paper only compared with the
literature values, and the experimental condition was quite
different. Zhang et al. found similar synergism outcome in
the extraction of Y in chloride medium [65]. They proposed a
theory for this system that no interaction with chloride ions
occur as opposed to the single system due to the increase of
steric hindrance, which resulted in the different complex
structure, RE(HA2)2(HB2) (A and B refer to the conjugate
base anion of EHEHPA and Cyanex272, respectively). Fig. 7 shows the transformation of the symmetry and stability of the
binary system complex which could improve the metal
selectivity and make the stripping process easier [27]. In
conclusion, the synergistic effect can only be achieved at low
acid concentration for the extraction of the selected LRE and
HRE pairs in Cyanex 272-EHEHPA system; at higher acidic
concentration, the synergism is concealed by the interaction
of the extracted complexes which give rise to antagonism
[15].
Table 6: Separation factor of some adjacent lanthanide in
individual and mixture of DEHPA and EHEHPA
Element DEHPA EHEHPA DEHPA
+EHEHPA
Ho-Dy 1.24 [19] 2.58 [19] 1.72 [62]
Y-Ho 1.24[27] 1.7 [26] 1.84[62]
Y-Er 1.1 [26] 1.2 [26] 1.32[62]
Tm-Er 1.50 [19] 1.33 [19] 3.04[62]
Yb-Tm 1.30 [19] 1.12 [19] 3.11[62]
Lu-Yb 1.03 [19] 1.13 [19] 1.82[62]
Since the 1950s, various extractants have been introduced
for the separation of RE. The weaknesses of these extractants
are still obscure although they possess many advantages such
as the attractiveness in the extraction efficiency and good
βfor the selected RE group. For example, EHEHPA, the most
common organophosphorus acid extractant, encounters several difficulties regarding saponification issue, low βfor
HRE, and complicated extraction process. Although higher
βcan be achieved by using more advanced extractant such as
Cyanex 272, there is the challenge of the increased cost and
difficulty in controlling the extraction to avoid
emulsification. Apart from that, the RE processes require a
large volume of non-polar solvent or diluent. Kerosene and
n-heptane are widely used in the industrial process and
standard for flammable and highly volatile solvent, which are
strictly enforced. The public concern was raised after one of
the RE processing factories in Mongolia stopped operating for several months due to a fire incident that burnt the factory.
The source of the fire was believed to come from the
kerosene in the processing plant [27]. In the effort to avoid
such incident, researchers have tried to replace the current
extractant and diluent with safer, non-volatile, and
non-flammable materials.
Fig. 4The proposed structure of A) single system of
Cl-EHEHPA/ Cl-Cyanex 272 and B) binary system of
Cl-EHEHPA-Cyanex 272 [65]
Past few years has been filled with various research on the
utilization of ionic liquid (IL) in the RE separation process. ILs are well-known for their fascinating properties, including
low volatility and combustibility [75], high ionic
conductivity as well as thermal stability [76]. Utilizing IL in
the separation process of REs could provide a new
perspective in the separation society as IL can be custom
designed according to the requirement. There are at least a
million binary ILs, and 1018 ternary ILs possible
combinations through the various arrangements of cation,
anion, and alkyl group that is attached to the cation head [77].
IL also has a unique attribute in the RE separation as it serves
as both solvent [78] and extractant [79]. There is also a combination of two functional groups in one IL. In
extraction, prevalent IL such as Cyphos 101 or Aliquat 336 is
known as functionalized ionic liquid (FIL) or task specific
ionic liquid (TSIL). This paper is interested in the recent
trend of ILs, which focuses on bifunctional ionic liquid
extractants (Bif-ILEs) by Aliquat 336, EHEHPA, and
DEHPA. Aliquat 336 is a quaternary ammonium IL, and it is
currently among the leading ILs with interesting properties of
low cost [80] and contain the least toxicity [81], [82]. Sun and
his team announced a new strategic approach in synthesizing
ammonium type of IL and creating a new series of IL containing Aliquat 336 such as [A336][CA-12],
[A336-CA100], [A336][P204], [A336][P507],
[A336][Cyanex272], and [A336][Cyanex302]. These
extractants are called Bif-ILEs due to the existence of the
dual functional group, which are cation and anion that have a
specific role in REMs extraction [83]. Sun also introduced
the extraction process that involves both cation and anion,
and it was given the term, “inner synergistic effect” [84]. The
synergistic effect of the system is significantly enhanced with
the involvement of both functional groups in the extraction
process.
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This unique inner synergistic effect offers greener
separation as the saponification of the wastewater can be
eliminated. It was reported that there were good efficiency
capabilities and high β in the RE extraction by [A336][P507]
and [A336][P204] [85]–[87]. The mechanism of Bif-ILE was
based on the formation of an extractable complex by [A336]+ cation, as well as [P507]- and [P204]- anion with RE3+ (Fig.
8). Sun et al. found that the D in [A336][P204] was 70 times
higher than their mixed precursor [84], A336+P204 due to
the strong interaction between Bif-ILEs and RE besides other
interactions for instance electrostatic, van der Waals, and
induction. [A336][P204] and [A336][P507] behave like a
neutral organophosphorus extractants [80]. In comparison to
EHEHPA and DEHPA as an extractant, the extraction of RE
in the Bif-ILEs system can be done in low acidity with good separation efficiency as the H+ was not involved.
Fig. 5Proposed structure and coordination of [A336][P204] with Eu3+[84]
Guo et al. found that [A336][P507] and [A336][P204]
have better separation for LRE in chloride media, and the
separation of HRE can be achieved in nitrate media [85]. The analyses in Table 3, Table 4, Table 7, and Table 8
demonstrate that [A336][P507] exhibits the highest βof LRE
in chloride media, especially for Pr-Nd and Sm-Nd with the
value of 9.52 and 4.70. There is a significant improvement in
the βof this adjacent LRE compared to [A336][P204],
EHEHPA, and DEHPA with the separation values of
(5.61,4.14), (1.17,2.00) and (1.06, 4.86), respectively. In
reality, the separation of Pr-Nd was among the toughest job in
the industry, and the values of the maximum βobtained from
other researchers were very low with β = ~1.5 [12], [19],
[47], [88], [89]. In addition, [A336][P204] dominates the
separation of HRE; from Er to Lu, followed by [A336][P507], and individual EHEHPA and DEHPA.
Although Bif-ILEs provide a better separation process than
the traditional extractant for REs, there are still other
emerging problems such as the formation of the third layer in
the extraction system [11], [90], rise in viscosity of the
organic phase [11], and cation deficit from shorter carbon
chain [90]. The third layer in Bif-ILEs system existed due to
the formation of macromolecules extractable complex that
could hardly dissolve in the organic phase. The complex
formed over the lone pair electron of P=O in the [P507]- or
[P204]- and unshared nitrogen from [A336]+. This third layer has a higher viscosity than the organic phase, and its volume
changed inversely with Bif-ILEs concentration [11].
However, Sun and Waters discovered an opposite
phenomena involving the third layer phase that appears when
the concentration of Bif-ILEs was maximum [90]. They
assumed that the layer appeared because of the partitioning of
the organic phase into two groups; high in metal-solvate
(lower layer) and rich in diluent (upper layer) based on
research published by Vasudeva dan Kolarik in 1996 [91].
Alternatively, the addition of isoctanol could eliminate the
third layer and the reduce the viscosity of the organic phase.
Isooctanol improves the solvating ability and quickly dissolved the complex in the organic solution [11].
IV. UNDERSTANDING SELECTION OF
EXTRACTANT IN INDUSTRY
The selection of an extractant in RE-SX system is highly depended on the target product and feed mixture. The
extractant chose in the primary SX separation affect the
separation sequence and selection of the other extractant in
the subsequent separation stage. Despite the number of the
published materials available on the extraction characteristic
of these organic extractants, the specific information of the
SX processes in the industry is well hidden.
Table 7: Separation factor of a) [A336][P507] and b)
[A336][P204] in chloride medium [85]
a) RE(III) Ce Pr Nd Sm
La 2.03 2.62 24.9 117
Ce 1.29 12.3 57.7
Pr 9.52 44.7
Nd 4.70
b) RE(III) Ce Pr Nd Sm
La 1.14 1.49 8.35 34.6
Ce 1.31 7.34 30.4
Pr 5.61 23.3
Nd 4.14
Table 8: Separation factor of a) [A336][P507] and b)
[A336][P204] in nitrate medium [85] a)
RE(III) Er Tm Yb Lu
Ho 5.63 18.1 78.1 83.5
Er 3.22 13.9 14.8
Tm 4.31 4.61
Yb 1.07
b) RE(III) Er Tm Yb Lu
Ho 6.93 23.3 184 1577
Er 3.36 26.6 227
Tm 7.92 67.8
Yb 8.55
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Subtopic IIdiscussed on the three types of SX system
namely, preconcentration SX, removal SX and separation SX
and their general relationship with the extractants.
Incorporating that information with the characteristic of
common extractants listed in Table 5, a good understanding
of the RE processing in the industry is achievable. The preparation of pure yttrium oxide (Y2O3) from Mountain Pass
bastnasite by Molycorp is one of the examples of utilization
of all three SX separation systems (Fig. 9) [92]. The feed is a
mixture of 50% Y2O3/REO, and the targeted products are
Y2O3 and a mixture of La-Er metals.
1) The SX1 is a preconcentration SX. The purpose of this
extraction cycle is to increase the amount of the Y2O3/REO in
the system from 50% to 95% using Aliquat – NO3 in Cyclosol
53. Aliquat – NO3 was chosen in this cycle probably due to
the ability of the extractant to separate the LRE and MRE. In
the SX1, the mixture of Y2O3 and HRE stayed in the aqueous
system and carried to the SX2.
2) The SX2 is a removal type SX. Versatic acid has shown an excellent capability to extracts the mixture with a high
concentration of yttrium and left the unwanted metals
/contaminants in the aqueous.
3) The SX3 is a separation SX, aimed to split the Y2O3 from
La, Yb and Lu mixture.
Extraction Scrubbing Stripping
Extraction Scrubbing
Stripping Scrubbing
Extraction
Aliquat – NO3
In Cyclosol 3Dil. HNO3
La-Er
Mg(NO3)2
Ytrrium nitrate solution
(50% Y2O3/REO)
Raffinate
Y, heavies and Mg
(95% Y2O3/REO)
Versatic acid
in keroseneHNO3 HNO3
La, Yb, Lu
Versatic acid
in kerosene
NH3 HNO3
Scrub liquor
Raffinate (Y2O3)
Fig. 6Flow diagram of recovery of pure yttrium oxide at Molycorp[92]
Other examples are monazite processing from the Indian
Rare Earth [4]. At the Alwaye plant, the operation to produce
various individual RE concentrate such as Sm, Eu, Gd, and Yis divided into several production stages. Right after the
chemical treatment, the mixed RE chloride solution
undergoes pre-concentration SX stage to produce Sm-Eu-Gd
concentrate from the scrubbing stage and Y concentrate from
the stripping stage (Fig. 10). The second extraction stage
aimed to separate the mixture of Sm-Eu-Gd concentrate into
individual element. As both extraction stages dealing with the
MRE, DEHPA is the best choice for the processes.
In the next stage, the RE chloride solution is extracted with
partially saponified PC88A (EHEHPA) in kerosene for its
ability to extract HRE. At the extraction stage, the MRE and
HRE are kept in the organic phase and the LRE is leaving through the raffinate (Fig. 11). Afterward, the MRE is
stripped selectively in the scrubbing cascade using 1 M HCl
and leaving the HRE to be stripped in the stripping cascade
by 3.5 M HCl. MRE in the strip solution is put through
another separation SX process to produce individual Gd, Sm
and LRE concentrate as shown in Fig. 12. The high
concentration of Sm and Gd in the feed solution along with 1
M DEHPA in kerosene as the extractant efficiently separate
them from the LRE mixture. The Sm concentrate is finally
stripped with 1.5 M HCl and 1 M DHEPA in two different
stages, and the Gd concentrate is recovered from the
stripping cascade with strong HCl solution. Overall, as expected, EHEHPA is employed in the process related to
HRE and DEHPA is preferably used in the separation of LRE
and MRE.
V. CONCLUSION AND FUTURE TREND
Undoubtedly, SX is the preferred technique in the past,
present, and in the nearing future as SX offers economic
advantages besides its ability to be easily adapted to a
continuous process, especially on an industrial scale.
Although multiple extractants have been proposed to be
applied in the SX system, these new extractants are still hard
to replace EHEHPA and DEHPA in the REMs
hydrometallurgical industry due to insufficient research data. The current trend showed that the best option for extracting
individual REM and HRE is by utilizing EHEHPA while
DEHPA is best for separating LRE from MRE. For the
minerals with a high concentration of Y, naphthenic acid is
preferred.
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Although this review could provide a guidelines in
selection of extractant, this is a complex process as there is
the need to evaluate another aspect such as starting materials,
composition, and target product. More research on systematic
structure-property studies should be conducted to understand
the behavior of this extractant. This study also has discovered the promising future
candidate in REs extraction, which is Bif-ILEs, as it could
perform the excellent separation. This extractant is a better
choice for industrial application due to the affordable market
price of IL, its physicochemical properties, extraction
capacity, selectivity, stripping and interfacial phenomenon.
Bif-ILE could diminish the risk of explosion of flammable
solvents in the traditional SX, and avoid saponified
wastewater crisis. Although this extractant has some
shortcomings as explained in the preceding section, it is believed that future research could find solutions in achieving
more advantageous separation for the industrial application.
Rare-earth chloride feed
(from monazite)
Extraction
Scrubbing
Sm-Eu-Gd concentrate
Reduction, precipitation
Extraction
Scrubbing
Stripping
Stripping
Gd concentrate
(~80% Gd2O3)
Sm strip solution
Reextraction
Sm concentrate
(~90% Sm2O3)
1 M HDEHP
Raffinate: La to Nd
Stripping 7.5 M HCl
Y concentrate:
(60% Y2O3)
Solvent recycle1 M HDEHP
2.5 M HCl
1 M HDEHP
2 M HCl
1.5 M HCl
5 M HCl
Eu recovery
Fig. 7 Flow diagram of MRE separation at Indian Rare Earth I [4]
Extraction
cascade
Scrubbing
cascade
Scrubbing
cascade
Stripping
cascade
Mixed RECl3
200 g/L
(1 vol.)
HCl 3.2 M
(0.49 vol.)
HCl 1 M
(0.23 vol.)
HCl 3.5 M
(0.07 vol.)
LRE concentrate
(La, Ce, Pr, Nd)
MRE concentrate
(Sm 45%)
HRE concentrate
(Y 60%)
Solvent regenerationPartially saponified PC88A
in kerosene (1.48 vol.)
Fig. 8 Flow diagram for the separation of mixed RE into LRE, MRE and HRE concentrate[93]
Selection of Extractant in Rare Earth Solvent Extraction System: A Review
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Extraction
cascade
Scrubbing
cascade
Scrubbing
cascade
Stripping
cascade
Sm concentrate
70 g/L (1 vol.)
HCl 2 M
(0.2 vol.)
HCl 1.5 M
(0.8 vol.)
HCl 5 M
(0.4 vol.)
LRE concentrate
(La, Ce, Pr, Nd)
Organic
scrubbing
cascade
Gd concentrate
(90%)
Solvent regenerationHDEHP 1 M in kerosene
(3.6 vol.)
Sm >95% pure
HDEHP 1 M (0.4 vol.)
Fig. 9 Flow diagram for the Sm recovery from the MRE concentrate using DEHPA [93]
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